1. Statement of the Technical Field
The inventive arrangements relate to methods for linearizing RF power amplifiers, and more particularly to a method for providing an envelope elimination and restoration (EER) amplifier with enhanced linearity.
2. Description of the Related Art
Power amplifiers are inherently nonlinear devices and are used in virtually all communications systems. Long-range communications require nigh-power amplifiers that are by far the most power consuming devices in the entire system. Amplifier efficiency is measured as the ratio of output power to the required DC power consumption. It is well known that very efficient power amplifiers are highly nonlinear. Highly efficient amplifiers are critical to mobile communications systems that require low power consumption for increased battery life and reduced size, such as cell phones, laptops, and military radios.
High-efficiency amplifiers with nonlinearities cause spectral re-growth (out-of-band noise), which leads to adjacent channel interference. They also cause in-band distortion, which degrades the bit-error rate (BER) performance for digital modulation waveforms. This degradation in performance has become a critical issue over the recent years as new emerging high-data rate digital waveforms are being used for transmission. Linearization is necessary to comply with FCC spectral mask requirements, to reduce BER, and to achieve acceptable amplifier efficiency, Predistortion can be used to convert a nonlinear amplifier into a linear amplifier.
Distortion associated with RF power amplifiers is often characterized by means of an amplitude-to-amplitude (AM-to-AM) modulation curve and an amplitude-to-phase (AM-to-PM) modulation curve. The AM-to-AM modulation curve shows the RF power amplifier gain as a function of the input power. The AM-to-PM modulation curve shows the output phase variation of the RF power amplifier as a function of the input power. It should foe understood that AM-to-AM distortion and AM-to-PM distortion can adversely affect the performance of an RF communication system,
Digital baseband predistortion is a cost effective technique that applies a correction to the input signal that is complementary to the distortion caused by the amplifier. The predistorter precedes the power amplifier (PA) and creates the inverse amplifier nonlinearity. A perfectly linear PA output would simply be a scalar multiple of the input signal to the PA (i.e. y=G*x, where G is the amplifier's gain). Because the amplifier “adds” distortion to the input signal, the output may he considered to he a scaled version of the input signal plus some distortion term (I.e. y=G*x+d). If the distortion term is time-invariant and can be accurately predicted in advance, the small gain theorem allows the distortion term to be cancelled by subtracting the distortion from the PA input (i.e. y=G*(x−d/G)+d=G*x).
Most power amplifiers exhibit some behavior of the small gain theorem, as the error term is repeatable using the same input signal. However, some power amplifiers also exhibit memory effects. This is shown as hysteresis in the AM-AM and AM-PM curves (i.e. different AM-AM curve for the rising slope versus the descending slope) and is most likely due to electro-thermal effects (e.g. heating/cooling of power semiconductor components) and/or significant group delay across the frequency hand. As such, the amplifier can no longer be modeled as memory less, where the present output depends only on the present input. Memory less predistortion for a PA with memory often results in poor linearization performance. Accordingly, there Is a need to address the effect of amplifier nonlinearities combined with memory effects.
One type of RF power amplifier which offers improved efficiency is the envelope elimination and restoration (EER) amplifier. EER amplifiers are well known in the art and can achieve very highly efficient conversion of DC energy to RF energy for complex waveforms having a varying envelope. They operate by separately processing the envelope and phase information contained in a modulated input signal. The phase information is communicated to a power amplifier where it is amplified as a constant envelope signal. This permits such phase information to be amplified using highly efficient non-linear amplifiers. The envelope information contained in the input signal is restored to the phase information after the signal has been amplified.
Highly efficient EER amplifiers using Class E topologies are known to have poor linearity. This poor linearity causes significant amounts of signal distortion. For example, such distortion often arises from pulse-width modulator circuits that are used to control the output envelope voltage, and from switching non-linearities which exist in the circuit used for amplifying the phase information. Most of the distortion products associated with an EER amplifier can be corrected by predistorting the input waveform. However, construction of a practical predistortion signal processor has proved to be difficult in the case of EER amplifiers used to amplify wideband signals.
Another technique for linearizing the performance of an RF power amplifier i known as feedforward linearization. With feedforward linearization, a 180 degree signal combiner is typically used to subtract a distorted version of a signal from an undistorted version of the signal. The result is an error signal. The error signal can be amplified and then subtracted from an RF power amplifier output signal in order to remove the error from the output signal.